[scudo] Remove fill when realloc to smaller size. (#191321)

In the reallocate function, when there is a realloc smaller than the
current size, the code would attempt to fill in the bytes after the new
size. This doesn't really add any extra security and is mostly a waste
of time, so skip it.

Remove the test that verifies this functionality.

GitOrigin-RevId: f0004d8143b346e0f5a13b4021f348165e27fb9a